Solid state imaging devices, including charge coupled devices (CCD), complementary metal oxide semiconductor (CMOS) imaging devices, and others, have been used in photo imaging applications. A solid state imaging device circuit includes a focal plane array of pixel cells or pixels as an image sensor, each cell including a photosensor, which may be a photogate, photoconductor, a photodiode, or other photosensor having a doped region for accumulating photo-generated charge. For CMOS imaging devices, each pixel has a charge storage region, formed on or in the substrate, which is connected to an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some CMOS imaging devices, each pixel may further include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference.
In a CMOS imaging device, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel reset and charge states. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
CMOS imaging devices of the type discussed above are generally known as discussed, for example, in U.S. Pat. Nos. 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524, and 6,333,205, assigned to Micron Technology, Inc.
Ideally, the digital images created by CMOS imaging devices are exact duplications of the light image projected upon the imaging sensor. However, various noise sources can affect individual pixel outputs and thus distort the resulting digital image. One such source is dark current, a current that appears as a photodiode signal even in the absence of light.
Dark current may result from many factors, including: leakage in the charge collection region of a photodiode; unwanted electrons from peripheral circuits and electron generation from infrared photons; current generated from trap sites inside or near the photodiode depletion region; band-to-band tunneling induced carrier generation as a result of high fields in the depletion region; junction leakage coming from the lateral sidewall of the photodiode; and leakage from isolation corners, for example, stress induced and trap assisted tunneling.
Reducing dark current in a photodiode is important in image sensor fabrication which use photodiodes as the photoconversion device described above. Methods for reducing dark current are especially important at high temperatures (e.g., greater than 50 degrees Celsius), because dark current increases exponentially with temperature. Additionally, because dark current may vary over the lifespan of an imaging device, these devices may become more susceptible to dark current influence and thus produce images of decreasing quality over time. Accordingly, a method to reduce dark current in imaging devices is needed.